1. Field of the Invention
The present invention relates to a method for producing printed wiring boards for mounting electronic components thereon and more particularly to a rigid-type multilayer printed wiring board which is excellent in economy and which ensures high-density surface mounting.
2. Description of the Prior Art
Printed wiring boards heretofore used have been so designed that a copper foil is firmly bonded to a paper-phenol board or glass-epoxy board and the necessary wiring is formed by subjecting the copper foil to an etching process or a plurality of such boards are laminated to produce the desired high-density assembly. While these printed wiring boards can be mass produced with resulting relative advantage in economy, they use resins as materials for the boards and therefore they are lacking in heat conductivity and heat dissipating properties. Thus, in the case of printed wiring boards on which components have been mounted by the high-density surface mounting, from the heat control point of view the tolerance limits are frequently reached and a particularly early solution of this problem is required in the case of power supply circuits and power electronics circuits using relatively high powers. In other words, it can be said that where the electronic components are miniaturized with a higher density and changeover to the surface mounting is simultaneously effected, the problem relating to the heat dissipating properties of the boards becomes universally important.
In an attempt to overcome the foregoing deficiency, it has been well known to use a printed wiring board employing a board of a ceramic material, e.g., alumina having a high heat conductivity. However, the use of this type of ceramic board is disadvantageous in that the production cost is two to three times higher than that of the previously mentioned printed wiring boards employing resinous boards and that their manufacture requires special processes. In addition, the ceramic board is subject to a change in size during the heating and firing and such deformation is unavoidable, thereby placing considerable limitations on the shape of the board.
On the other hand, a printed wiring board is known in which a copper foil is bonded to an aluminum board through an insulation laminate and unwanted portions of the copper foil are removed by etching, thereby producing the printed wiring board. However, the greater part of printed wiring boards of this type are single-layer single-sided printed wiring boards and thus there still exists the disadvantage of failing to fully accomplish the desired high-density mounting.
To form a printed wiring on each side of a metal board, e.g., aluminum board, it is of course essential to electrically insulate the through-hole paths and the metal board from each other and therefore not only the production process is complicated but also serious problems must be solved with respect to the electrical reliability of the through-hole portions.
On the other hand, a method is proposed, for example, in Japanese Patent Laid-Open No. 58-9399 for producing a metal-cored two-layer printed wiring board having a feature that double-sided printed wiring patterns including through-hole paths are first formed on a flexible insulating film and then the film is bonded onto a metal board through an insulation laminate. However, in the case of this method, e.g., a two-layer printed wiring board production method in which a flexible printed wiring film formed with the necessary wiring is bonded onto a metal board, the flexible insulating film holding the printed wiring is stretched by the heat and the pressing pressure which is applied during the bonding of the flexible insulating film to the metal board and deformation or damage to the printed wiring frequently occurs, thereby making it impossible to produce a multilayer printed wiring board which is high in accuracy and reliability.